superset/import_dashboards.html: Update title, clean up html (#4972)
superset/import_dashboards.html: Update title, clean up html (#4972)

* Changes "Import the dashboards." to "Import dashboards"
* Cleans up the HTML to add quotes, self close tags, etc.
* Adds a class to the `<submit>` button to utilize bootstrap style
* Remove the `<title>` tag in body as it's not vaild HTML and redundant with `{% block %}`
apache/incubator-superset,airbnb/superset,dmigo/incubator-superset,dmigo/incubator-superset,airbnb/superset,mistercrunch/panoramix,apache/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,apache/incubator-superset,airbnb/caravel,timifasubaa/incubator-superset,apache/incubator-superset,apache/incubator-superset,zhouyao1994/incubator-superset,airbnb/caravel,mistercrunch/panoramix,airbnb/superset,timifasubaa/incubator-superset,mistercrunch/panoramix,timifasubaa/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,airbnb/caravel,airbnb/superset,dmigo/incubator-superset,timifasubaa/incubator-superset,dmigo/incubator-superset,airbnb/caravel,mistercrunch/panoramix